I’ve observed if I say nothing (because I simply don’t know how to react), opinionated people think you agree with them, which I don’t. I don’t care.

what I want to tell him, next time he starts ranting: ‘I don’t care what you think, leave me alone’.

However, I may have to work with this person in the future, so what about ‘you don’t have to tell me everything you think, most of the time I don’t pay attention’ and if he keeps pushing it ‘it’s tiring working with a person who has to rant to feel good, it’s boring and makes me ignore you, which is a problem, because we work together.’ And leave.

What about ‘everybody has problems, maybe talk to a therapist? I cannot help you’.

Or maybe simply leaving when he starts his rants?